1. Field of the Invention
The present invention generally relates to a heat developable photosensitive material, and in particular to a heat developable photosensitive material having high sensitivity and high heat development activity, providing a high image density even if the silver content thereof is small, being excellent in image preservation, and showing low fogging.
2. Description of the Related Art
Reduction of the amount of waste which is produced when films for medical diagnosis and photographic printing are processed has been increasingly required in recent years in view of environmental preservation and for saving work spaces. Accordingly, technologies related to heat developable photosensitive materials are required for providing films for medical diagnosis and photographic printing. Such films can be efficiently exposed to light by laser image setter or laser imager to form clear and black images with high resolution and sharpness. No liquid chemicals are needed when the heat developable photosensitive material is processed, making it possible to supply to customers a heat development system that is more simple and friendly to the environment.
While similar advantages are required in general purpose imaging materials, cold black-tone images are preferable, particularly for use in medical diagnosis, when considering availability for easy diagnosis, since high image quality with excellent sharpness and granular properties is necessary for attaining the required fine depiction. Although various hard copy systems taking advantage of pigments and dyes, such as an ink-jet printer and electrophotography, are commercially available today as general imaging systems, none of them are satisfactory as output systems for medical images.
Examples of the heat imaging system utilizing organic silver salt are disclosed in U.S. Pat. Nos. 3,152,904 and 3,457,075, and described in “Thermally Processed Silver System”, Imaging Processes and Materials, Neblette Vol. 8, written by D. Klosterboer, edited by J. Sturge, V. Walworth and A. Shepp, Capt. 9, p 279, 1989.
Generally, the heat developable photosensitive material has a photosensitive layer in which a photocatalyst (e.g., silver halide), a reducing agent, a reducible silver salt (e.g., an organic silver salt) and, if necessary, a tone control agent that controls tone of silver is dispersed in a binder matrix.
The heat developable photosensitive material is heated at a high temperature (for example 80° C. or more) after imagewise exposure to cause an oxidation-reduction reaction between the reducible silver salt which functions as an oxidizing agent and a reducing agent and to form a back silver image. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image of silver halide generated by exposure. Consequently, the black silver image is formed in the exposed area. These procedures are disclosed in many references such as U.S. Pat. No. 2,910,377 and Japanese Patent Application Publication (JP-B) No. 43-4924.
In the heat developable photosensitive material, it is preferable that the oxidation-reduction reaction between the reducible silver salt and the reducing agent is proceeded at a feasibly acceptable temperature and reaction time in order to obtain an acceptable image density. However, there is a still need for the heat developable photosensitive material having a high sensitivity and high heat development activity, providing high image density even if a silver content thereof is small, being excellent in image preservation, and showing low levels of fogging.
In particular, reduction of the amount of the silver contained in the photosensitive material is desired in view of improved image stability against light after the development process.
Since no fixing process of the organic silver salt is employed in the heat developable photosensitive material using the organic silver salt, silver images may appear by light or heat after forming the silver image by heating. Such unnecessary images are not formed under ordinary conditions of use. However, when the preservation conditions are very severe for the heat developable photosensitive material, for example when processed films are placed in a hot vehicle during summer for transportation, troubles such as color changes throughout the film or transfer of characters printed on a bag as a container of the film onto the film, or a so-called fogging, may occur.